A coupler of this type is known, as described in French patent No. 2,676,548, filed under the name of the present applicant (corresponding to U.S. Pat. No. 5,222,167). According to the teachings of this document, the amount of axial space required by the coupler is limited through the elimination of all points of inflection on the guides except at locations where they are necessary, i.e., at the junctions. In this way it was possible to create a coupler with 16 outputs and a reduced space requirement that nevertheless met the necessary specifications in terms of the uniformity of the distribution of the light power transmitted to the various output ports, and in terms of achromaticity in the two wavelength domains used in fiber-optic telecommunications.
However, when the type of waveguide-pattern design rules described in the above-mentioned patent are used to create a coupler with a larger number of output ports, such as 32, difficulties are encountered in terms of meeting the specifications. These difficulties fall into two basic classes.
First, the pattern in question consists only of concave waveguide branches. In order to create a coupler with 32 output ports, obviously the number of branches must be multiplied, with a resulting loss of accrued curvature. It is also known that, particularly with single-mode waveguides, concave branches cause mode misalignments that are unfavorable to a balanced distribution of the light power between the two output branches of a junction.
Second, the pattern consists essentially of Y-junctions whose axes are inclined in relation to the axis of the coupler. This arrangement also involves an arrangement which, when the number of junctions increases, is unfavorable to the uniformity of the distribution of the light power among the various outputs. In fact, in the classical procedure, the waveguides in the coupler are obtained by means of ion-exchange with a glass substrate, for example, through a mask. When the aid of a computer is employed in creating the mask, as is commonly done in current practice, the mask is drawn and defined on a surface that is cut out into square image elements that are arranged in lines that are parallel to the axis of the coupler and in columns. If such image elements are used to design junctions that are not parallel to the axis of the coupler, then the edges of the resulting waveguides, which are usually deformed by a notching or crenelation due to the image elements used, display further asymmetrical crenelations which themselves can in turn introduce corresponding asymmetries into the propagation of the light through the waveguides that are obtained with such a mask.
Therefore, the goal of the present invention is to create an integrated optical coupler with one input port and 2.sup.n output ports, such as 32, that has the uniformity in terms of the distribution of light power and the achromaticity that are required for applications in the telecommunications field.